Measurement uncertainty in VNAs vs. TDRs, Part 1

In the test and measurement industry, two distinct camps exist: those who favor vector network analyzers (VNA) and those who favor time domain reflectometers (TDR). Each camp relies heavily upon its instrument of choice for a variety of test and measurement and analytical tasks. The TDR’s strong suit is temporal analysis — characterizing impedance or reflection coefficient with respect to time. Its quick setup, intuitive controls, and results-oriented operation appeal to a broad range of users. The VNA, on the other hand, excels in frequency domain analysis — characterizing amplitude and phase with respect to frequency. Learning to operate the VNA can be complex, but it offers an extremely stable, precise, and versatile measurement platform. Interestingly, both instruments have the ability to perform time or frequency domain analysis through built-in Fast Fourier Transform (FFT) algorithms or ancillary software.

Individuals working in digital applications tend to prefer the TDR, while those involved in traditional RF applications consider the VNA to be a laboratory staple. The push for ever-faster data rates has fueled an analytical rethinking of high-speed digital signaling. Contemporary wisdom views high-speed digital systems as high-frequency applications; therefore more traditional, physics-based microwave analysis techniques can be applied. Once this concept is embraced, users follow a tendency to exploit the strengths of the TDR and the VNA, combining time and frequency domain analysis to accelerate design and development cycles. Both instruments can measure impedance, time delay, phase delay, and reflection coefficient, so they are often thought of as equals. This begs the question: Is there a quantifiable difference in measurement uncertainty between the TDR and VNA?

Setting Up a Performance Comparison
Characterizing the time delay of a passive device, such as a coaxial cable assembly, is a common use for the TDR and VNA. It is therefore an ideal vehicle for a performance comparison. How do the two compare under ideal test conditions and the less-than-ideal environment of production testing? Do both instruments possess similar levels of measurement precision?

At W. L. Gore & Associates, Inc. (Gore), we addressed these questions by examining the measurement uncertainty and repeatability of the TDR and VNA. These tests did not, however, address the absolute measurement accuracy of either instrument. In the first series of experiments, we tested six cable assemblies (also referred to as the device under test, DUT) on both a TDR and a VNA. We then ran another series of experiments using the best-performing cable assembly from the first series to evaluate the best-case scenario. Finally, to ensure TDR/VNA test parity, VNA measurements of the best-performing cable assembly were made using one-port s11 reflection techniques in addition to the more traditional two-port s21 transmission method.

Description of Multiple-Assembly Experiment
Objective: Measure the time delay of the cable assemblies with both a TDR and a VNA in a manner consistent with commonly used production test practices, and compare the resulting measurement uncertainty of the two instruments under these conditions.

To understand the capabilities of any measurement system, it is important to test the system’s response to a variety of inputs. Data based upon a single type of input can lead to erroneous conclusions. Therefore, we designed the experiment to use different cable assembly types (the DUT) with a range of insertion loss and voltage standing wave ratio (VSWR) characteristics made by various manufacturers. Six new cable assemblies were used, each equipped with SubMiniature version A (SMA) pin connectors. Electrical data was acquired through VNA analysis (Table 1).

The experiment consisted of two rounds of testing. Within a round, each sample was connected to the TDR or VNA and measured five consecutive times without being disconnected or disturbed (repeat testing). After five measurements, the sample was removed from the instrument and not reconnected until the next round of testing (round testing). Sample assemblies were labeled 1 through 6, and their test order within each round was randomized to reduce test bias. The same operator was used throughout the entire experiment. Tests were conducted over a two-day period: TDR testing on the first day, VNA testing on the second day.

In total, there were 60 measurements: six samples x five repeat tests x two rounds. Repeat testing was intended to capture the instrument’s test repeatability or instrument uncertainty. The round-to-round testing was designed to reveal measurement reproducibility, but also indirectly captured connect/disconnect, test fixture, and to some extent, operator influences. In summary, round testing of a sample reflects instrument uncertainty, while round-to-round testing reflects test uncertainty.

Test Configurations
During the TDR portion of testing, the sample assemblies were connected directly to the TDR sampling head, while the opposite end was terminated with a 3.5mm precision open standard. (Figure 1) This was done to ensure a well-defined and controlled termination. Once an assembly was disconnected from the TDR, the precision open was removed as well, and it was connected to the next sample ready for testing.

In the VNA portion of testing, the sample assemblies were connected between ports 1 and 2 (Figure 2).

In both TDR and VNA testing, the samples were well-supported. Standard RF cable assembly care and handling practices were exercised, e.g., cleaning of connectors with alcohol, drying with a moisture-free air source, tightening connectors to proper torque, and careful handling of the cable itself.